1. Field of the Invention
The present invention relates to a lens having a zooming function by changing focal length to change a field angle, particularly, a zoom lens preferable for a digital camera, a video camera, and the like acquiring digital image data of a subject by use of a solid-state image pickup device, an imaging apparatus having such a zoom lens as a photographic optical system, and an information device having such a photographic function.
2. Description of the Related Art
Recently, in widely used digital cameras, various types of requirement, especially high performance and small size have been further required and therefore a high-performance and small-size zoom lens which is installed therein as a photographing lens is also required.
In the zoom lens, in order to reduce the size, it is required to reduce an entire length (distance from a most object side lens surface to an imaging plane) in use, and it is important for reduction of size to reduce an entire length in a collapsed state by reducing a thickness of each lens group. As the high performance of the zoom lens, a resolution corresponding to an image pickup device having at least ten to fifteen million pixels is required for an entire zooming range.
Furthermore, a wider field angle of a photographing lens is required by many users and a half field angle of the zoom lens at a wide angle end is preferably 38 degrees or more. The half field angle of 38 degrees corresponds to a focal length of 28 mm in a case of a silver-salt camera using a silver-salt film (so-called Leica film) having a width of 35 mm.
Furthermore, high magnification ratio is also required. The zoom lens having a focal length corresponding to of 28 to 200 mm in a 35 mm silver salt camera conversion (about 7.1 times) enables all of the general photographing.
As the zoom lens for the digital camera, various types of the zoom lenses are used. As a well known zoom lens having high magnification ratio or large diameter, there is a zoom lens including four lens groups of a first lens group having a positive refractive power, a second lens group having a negative refractive power, a third lens group having a positive refractive power, and a fourth lens group having a positive refractive power, which are disposed in order from an object side. In such a zoom lens, when changing the magnification from the short focal end to the long focal end, an interval between the first lens group and the second lens group is increased, an interval between the second lens group and the third lens group is decreased, and an interval between the third lens group and the fourth lens group is changed.
As the zoom lens of this type, the first lens group is fixed or reciprocated and moved in an arc which is convex at an image side when changing the magnification. In this type, if a movement amount of the second lens group which largely shares a function of the magnification operation is attempted to be largely ensured, an aperture stop provided at a vicinity of the third lens group is far from the first lens group even when the zoom lens is at the short focal end. Accordingly, when the wide angle zoom lens is required, the first lens group is required to be large in order to achieve the high magnification ratio. Therefore, in order to achieve a wide angle, high magnification and small size zoom lens, the first lens group is preferably moved so as to be positioned at the long focal end to the position which is at the object side of the position at the short focal end.
The entire length of the zoom lens at the wide angle end is reduced so as to be less than the entire length of the zoom lens at the telephoto end so that the size of the first lens is prevented from increasing and a sufficient wide angle can be achieved.
On the other hand, in a correction of the chromatic aberration which readily occurs with increasing the magnification ratio and the focal length, it is effective to use the lens having an anomalous dispersion property, as well known.
As the zoom lens including a first lens group having a positive focal length, a second lens group having a negative focal length, a third lens group having a positive focal length, and a fourth lens group having a positive focal length, which are disposed in order from an object side, and in which an interval between the first lens group and the second lens group is increased, an interval between the second lens group and the third lens group is decreased, and an interval between the third lens group and the fourth lens group is changed when changing the magnification from the short focal end to the long focal end, the zoom lens using an anomalous dispersion property is disclosed in Japanese Patent Application Publication Nos. H08-248317, and 2008-026837 and Japanese Patent Nos. 3391342 and 4405747.
In these Documents, in the zoom lens disclosed in Japanese Patent Application Publication No. 1108-248317, the first lens group is fixed when changing the magnification and half field angle at the short focal end is 25 degrees and a wide angle is not sufficient.
The zoom lens disclosed in Japanese Patent No. 3391342 includes a four-lens group configuration having positive-negative-positive-positive lens groups (see Examples 1, 2 and 6) and the zoom lens has a field angle of about 29 to 32 degrees at the short focal end so that the wide angle is not sufficiently achieved.
The zoom lens disclosed in Japanese Patent No. 4405747 has a wide angle, that is, a half field angle of about 37 degrees at the wide angle end. However, the lens configuration includes many lenses, that is, 14 lenses and therefore it is difficult to achieve the reduced entire length in a collapsed state and low cost.
The zoom lens disclosed in Japanese Patent Application Publication No. 2008-026837 has a relatively small number of lenses, that is, 9 to 11 lenses so that wide angle and high magnification is achieved with a simple configuration. However, there is room for improvement in the small size because the entire length at the telephoto end is slightly large.